An attenuator is a device used to reduce the power level of a signal without introducing any appreciable distortion. In particular, attenuators provide amplitude control for high frequencies such as microwaves. In many microwave systems, microwave radio frequency (RF) attenuators are required for automatic gain control of both receiver and transmitter systems. Microwave RF attenuators are available in many formats including fixed-value attenuators, continuously variable attenuators, and digital attenuators.
Digital attenuators have a distinguishing feature in that a known amount of attenuation can be selected at high speed according to the state of a logic signal. Digital attenuators vary the strength of input signals in response to digital control signals and can switch in discrete, finite attenuation states. For example, in a typical 1-bit digital attenuator, the amount of attenuation, or the “attenuation step,” offered by the attenuator varies depending on whether the bit of the control signal has a value of “0” or “1.” Typically, if a 2-bit or other multiple bit digital attenuator is desired, a plurality of 1-bit digital attenuators are cascaded to produce the desired m-bit digital attenuator (where m>2).
In order to achieve increased attenuation accuracy while maintaining a fixed attenuation amount, smaller and more accurate attenuation steps are required for greater resolution. This requires multiple bits that are sensitive to fine adjustment and relatively insensitive to temperature and incident power. As the number of bits increase, however, distortion and insertion loss also increase, which is disadvantageous in many systems. Accordingly, it is an object of this invention to satisfy the need for improved multi-bit digital attenuators having fine resolution and accurate attenuation capabilities with minimal distortion and insertion loss.